Magnetic memory element and magnetic memory

ABSTRACT

According to one embodiment, a magnetic memory element includes a first magnetic layer, a second magnetic layer, a nonmagnetic layer disposed between the first magnetic layer and the second magnetic layer, an electrode disposed on a side surface of the first magnetic layer, and a first insulation layer disposed between the first magnetic layer and the electrode, and including a first region with a first film thickness and a second region with a second film thickness which is less than the first film thickness.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2015-053903, filed Mar. 17,2015, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a magnetic memoryelement and a magnetic memory.

BACKGROUND

An MRAM (Magnetoresistive Random Access Memory) is a magnetic memorywhich utilizes the reversal of magnetization. A spin-transfer MRAM,which uses write by spin transfer, is excellent in high speedperformance, high integration density, and durability, and isprospective as a general-purpose nonvolatile random access memory.

In the spin-transfer MRAM, an MTJ (Magnetic Tunnel Junction) element isused as a memory element. This MTJ element includes a storage layerincluding a magnetic layer, the direction of magnetization of which isvariable by a write operation of the memory; a reference layer includinga magnetic layer, the direction of magnetization of which is fixed inone direction; and a tunnel barrier layer which is interposed betweenthe storage layer and the reference layer and forms a tunnel barrier.Depending on whether the magnetization of the storage layer and themagnetization of the reference layer are in a parallel state or in anantiparallel state, the electrical resistance in a case of causing anelectric current to flow in a direction perpendicular to the filmsurface of the MTJ element takes a low resistance state or a highresistance state. By using a difference in resistance between theparallel state and the antiparallel state, data (information) can beread from the MTJ element.

In the write by spin transfer, the magnetization of the storage layer isreversed by causing an electric current to flow in a directionperpendicular to the film surface of the MTJ element. For example, whenthe magnetization is reversed from the antiparallel state to parallelstate, an electric current is caused to flow in such a direction thatelectrons flow from the reference layer to the storage layer. Thedirection of the electric current becomes, conversely, a direction fromthe storage layer toward the reference layer. By this current flow, aspin torque acts on the magnetization of the storage layer such that themagnetization of the storage layer becomes parallel to the magnetizationof the reference layer, and the magnetization of the storage layer canbe reversed by causing a current of a predetermined threshold or more toflow. On the other hand, when the magnetization is reversed from theparallel state to antiparallel state, an electric current is caused toflow in such a direction that electrons flow from the storage layer tothe reference layer. By this current flow, a spin torque acts on themagnetization of the storage layer such that the magnetization of thestorage layer becomes antiparallel to the magnetization of the referencelayer. In this manner, by changing the direction of the electric currentthat is caused to flow, data rewrite is enabled.

In the MRAM using spin-transfer writing, an electric current is appliedto the MTJ element through the same path at a time of read and at a timeof write. Thus, there is, potentially, a risk of read disturb by whichdata is rewritten at a time of read. In order to avoid this risk, thereis a method of setting a read current, which is supplied to the MTJelement at a time of read, to be lower than a write current which issupplied to the MTJ element at a time of write. By this technique, theprobability of occurrence of read disturb is reduced. However, todecrease a read current causes a decrease in read sensitivity. Thus,there is a lower limit to a practical read current.

This being the case, in order to avoid the occurrence of read disturb,such a method has been proposed that the probability of occurrence ofread disturb is reduced by setting the pulse width of a read current tobe smaller than the pulse width of a write current. However, in a memorywhich requires a high speed operation, the pulse width of a writecurrent becomes smaller due to the demand for an increase in speed ofthe write operation. It is thus necessary to make smaller the pulsewidth of the read current, but there is also a lower limit to the pulsewidth of the read current, because of problems of read sensitivity andwiring delay of current pulses.

Furthermore, it has been reported that the write current increases ifthe pulse width of the pulse of the write current is decreased in orderto meet a demand for an increase in speed at a time of write. Thus, asregards the reduction in write power, that is, power saving, animportance is placed on the reduction in write current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for explaining the principle of suppression of anenergy barrier that is necessary for magnetization reversal, by applyinga voltage to a side surface of a magnetic layer having magnetizationperpendicular to a film surface;

FIG. 2 is a view illustrating a relationship between a film thickness ofa side-wall insulation film and a variation amount of a magnetizationreversal energy barrier;

FIG. 3 is a cross-sectional view of an MTJ element according to each ofembodiments;

FIG. 4 is a cross-sectional view taken along a plane parallel to a filmsurface of a storage layer in FIG. 3;

FIG. 5 is a view of a circular MTJ element according to a firstembodiment;

FIG. 6 is a view of a rectangular MTJ element according to the firstembodiment;

FIG. 7 is a view of a rectangular MTJ element according to a secondembodiment;

FIG. 8 is a view of a circular MTJ element according to the secondembodiment;

FIG. 9 is a view for explaining a memory cell array of a magnetic memoryaccording to a third embodiment;

FIG. 10 is a view for explaining a disposition of a high electric fieldregion in the memory cell array of the magnetic memory according to thethird embodiment;

FIG. 11 is a view for describing a forming method (1) of a side-wallinsulation film of an MTJ element according to the third embodiment;

FIG. 12 is a view for describing the forming method (1) of the side-wallinsulation film of the MTJ element according to the third embodiment;

FIG. 13 is a view for describing a forming method (2) of the side-wallinsulation film of the MTJ element according to the third embodiment;

FIG. 14 is a view for describing the forming method (2) of the side-wallinsulation film of the MTJ element according to the third embodiment;

FIG. 15 is a view for describing the forming method (2) of the side-wallinsulation film of the MTJ element according to the third embodiment;

FIG. 16 is a view for describing a control electrode of an MTJ elementaccording to a fourth embodiment;

FIG. 17 is a view for describing the control electrode of the MTJelement according to the fourth embodiment;

FIG. 18 is a view for describing the film thickness of a side-wallinsulation film of an MTJ element according to a fifth embodiment;

FIG. 19 is a cross-sectional view of memory cells of a magnetic memoryaccording to a sixth embodiment;

FIG. 20 is a view for describing the memory cells of the magnetic memoryaccording to the sixth embodiment;

FIG. 21 is a view illustrating examples of potentials of bit lines andcontrol lines at times of write and read in the magnetic memoryaccording to the sixth embodiment; and

FIG. 22 is a circuit diagram of the magnetic memory according to thesixth embodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a magnetic memory elementincludes a first magnetic layer, a second magnetic layer, a nonmagneticlayer disposed between the first magnetic layer and the second magneticlayer, an electrode disposed on a side surface of the first magneticlayer, and a first insulation layer disposed between the first magneticlayer and the electrode, and including a first region with a first filmthickness and a second region with a second film thickness which is lessthan the first film thickness.

Various embodiments will be described hereinafter with reference to theaccompanying drawings. In the description below, common parts aredenoted by like reference numerals throughout the drawings.

[1] Principle

Such a phenomenon is known that, in a structure in which top and bottomelectrodes are disposed on the upper and lower parts of a multilayerstructure of a magnetic layer and an insulation film, if a voltage isapplied to these top and bottom electrodes, magnetic anisotropy energyin the direction of the voltage application varies. This phenomenonoccurs by an interaction between an electric charge, which is inducednear an interface between the magnetic layer and insulation film, andspin-polarized electrons of the magnetic layer.

The inventors first discovered that, in a structure in which anelectrode is disposed, with an insulation film interposed, on a sidesurface of a magnetic layer whose magnetization is perpendicular to afilm surface, an energy barrier of magnetization reversal can becontrolled with high efficiency by applying a voltage between themagnetic layer and the electrode. Here, the “film surface” refers to anupper surface of the magnetic layer. This principle will be explainedwith reference to part (a) of FIG. 1 to part (d) of FIG. 1.

Part (a) of FIG. 1 illustrates a simplified model for explaining theprinciple. As illustrated in part (a) of FIG. 1, this model includes adiscoidal magnetic layer 2; an insulation film 4 surrounding a sidesurface of the magnetic layer 2; and a control electrode 6 disposed on aside opposite to the magnetic layer 2, with the insulation film 4 beinginterposed. The magnetic layer 2 is a layer simulating the storage layerof an MTJ element (magnetic memory element). A voltage is appliedbetween the control electrode 6 and magnetic layer 2. For example, apower supply 8 is connected to the control electrode 8, and thepotential of the magnetic layer 2 is fixed at zero potential. It isassumed that magnetization 3 of the magnetic layer 2 is a macro-spinmodel in which all magnetization directions are identical.

Part (b) of FIG. 1 is a horizontal cross section of the modelillustrated in part (a) of FIG. 1, and illustrates a case in which themagnetization 3 is in a direction within the film surface (hereinafter,also referred to as “in-plane direction”). Since the axis of easymagnetization of the magnetic layer 2 is a direction perpendicular tothe film surface, the magnetization 3 of part (b) of FIG. 1 indicates astate in which the magnetization 3 is in a direction of the axis of hardmagnetization. However, since the shape of the magnetic layer 2 isdiscoidal, if the magnetization 3 is in the in-plane direction, themagnetization 3 is equivalent in terms of energy in any of directionswithin the plane.

As illustrated in part (b) of FIG. 1, the center of the disc is set tobe the origin, an x axis and a y axis are disposed within the filmsurface, and the direction of the magnetization 3 is set to be an xaxis. If a voltage is applied between the control electrode 6 andmagnetic layer 2, an electric charge is induced at an interface betweenthe insulation film 4 and magnetic layer 2 in accordance with theapplied voltage, and magnetic anisotropy energy in a perpendiculardirection to the interface varies.

For example, in a multilayer structure including an insulation film,which includes MgO, and a magnetic layer which includes Fe and whosemagnetization is in the in-plane direction, if a negative voltage isapplied to the electrode that is disposed on the side opposite tomagnetic layer with respect to the insulation film, the magneticanisotropy energy in the direction perpendicular to the interfaceincreases and a more stable state occurs in the direction perpendicularto the interface. In addition, if a positive voltage is applied to theabove-described electrode, the magnetic anisotropy energy in thedirection perpendicular to the interface decreases and a more stablestate occurs in the direction parallel to the interface, i.e. thedirection perpendicular to the normal of the interface.

It is known that, also in a MTJ element having a multilayer structurewith a perpendicular magnetic anisotropy including CoFeB, MgO and CoFeB,the magnetic anisotropy energy varies by similarly applying voltages.Specifically, the perpendicular magnetic anisotropy energy of thestorage layer decreases by applying a negative voltage to the electrodedisposed on the storage layer side with respect to a tunnel barrierlayer formed of MgO, and applying a positive voltage to the electrodedisposed on the reference layer side. In addition, the perpendicularmagnetic anisotropy energy of the storage layer increases by applying apositive voltage to the electrode disposed on the storage layer side,and applying a negative voltage to the electrode disposed on thereference layer side.

It is also known that, in a multilayer film of a magnetic layer and aninsulation layer, the magnetic anisotropy energy in the directionperpendicular to the interface increases by applying a positive voltageto the electrode disposed on the insulation layer side. For example,consideration is now given to the case in which a voltage was applied toa multilayer film (MgO/FePd) of a magnetic layer which includes FePd ofan L1₀ structure with magnetization perpendicular to the film surface,and an insulation layer formed of MgO. If a positive voltage is appliedto the electrode disposed on the side of the insulation layer of MgO inrelation to the electrode disposed on the side of the magnetic layer ofFePd, the perpendicular magnetic anisotropy energy increases, and,conversely, if a negative voltage is applied, the perpendicular magneticanisotropy energy decreases.

In the model illustrated in part (a) of FIG. 1, the insulation film 4 isdisposed in a manner to cover the side surface of the magnetic layer 2.Thus, a variation amount of magnetic anisotropy energy of the entiretyof the magnetic layer 2 can be calculated by integrating the variationamounts of magnetic anisotropy energy in minute regions of respectivepoints of the interface between the magnetic layer 2 and insulation film4 with respect to the entire side surface of the magnetic layer 2.

It is now assumed that the variation amount of magnetic anisotropyenergy per unit area of the interface, in a case in which the directionof magnetization is perpendicular to the interface, is K_(s). Asillustrated in part (b) of FIG. 1, an angle formed between a normalvector 5 of the interface and the direction of magnetization is θ at apoint in a direction of azimuth angle θ to the direction ofmagnetization (x-axis direction). Thus, a variation amount δE_(s) ofmagnetic anisotropy energy due to an electric field, which is applied tothe minute region of the interface at this point, is expressed byδE _(s) =K _(s) cos² θδS  (1)where δS is an area of the minute region of the interface.

As indicated by the following equation (2), by integrating this energyvariation amount δE_(s), a variation amount E_(s) of magnetic anisotropyenergy due to an applied voltage is obtained.

$\begin{matrix}{E_{S} = {\frac{1}{2}K_{s}S}} & (2)\end{matrix}$where S is an area of the entire interface.

On the other hand, part (c) of FIG. 1 illustrates a state in which themagnetization of the magnetic layer 2 of the model is directed in aperpendicular direction which is an easy axis. In this case, since theangle θ formed by the normal vector 5 and magnetization 3 is a rightangle, the value of equation (1) becomes zero at all points of theinterface. Thus, the variation amount E_(s) of magnetic anisotropyenergy, which was integrated by the entirety of the interface, alsobecomes zero.

Part (d) of FIG. 1 is a view in which the potential energy of themagnetic layer 2 is schematically plotted relative to the direction ofmagnetization. Two states, in which the potential energy takes a minimumvalue, represent states in which the magnetization 3 is perpendicular tothe film surface and is set in the upward and downward directions. Astate, in which the potential energy takes a maximum value, represents astate in which the magnetization 3 is set in the direction of the filmsurface. The energy barrier of magnetization reversal becomes adifference between these potential energies.

A solid line 10 indicates a potential energy in a case in which thepotential of the control electrode 6 is equal to that of the magneticlayer 2, and an energy barrier that is necessary for reversal under thiscondition is indicated by ΔE. On the other hand, a chain line 11 and achain line 12 indicate potential energies in states in which a voltageis applied between the magnetic layer 2 and control electrode 6.

When the variation amount E_(s) of magnetic anisotropy energy due to anapplied voltage is positive, the maximum state of potential, in whichthe magnetization is set in the in-plane direction, becomes more stablethan in the case in which no voltage is applied. Thus, as indicated bythe chain line 12, the maximum value of potential energy decreases by|E_(s)|. Conversely, when the variation amount E_(s) is negative, themaximum state of potential becomes unstable. Thus, as indicated by thechain line 11, the maximum value of potential energy increases by|E_(s)|. By applying the voltage in this manner, the maximum value ofpotential energy varies by |E_(s)|.

On the other hand, as described with reference to part (c) of FIG. 1,the minimum value of potential energy does not vary by the applicationof a voltage. As a result, by applying a voltage to the interface, theenergy barrier ΔE of magnetization reversal varies. When a voltage wasapplied with such a polarity that the perpendicular magnetic anisotropyenergy at the interface between the magnetic layer 2 and insulation film4 increases, ΔE decreases. When a voltage was applied with such apolarity that the perpendicular magnetic anisotropy energy at theinterface decreases, ΔE increases. The absolute value of this variationamount becomes equal to the value given by equation (2).

In the meantime, as is understood from part (d) of FIG. 1, that theenergy barrier ΔE of magnetization reversal of the storage layerincreases means that the magnetization of the storage layer becomesdifficult to be reversed. Thus, this energy barrier ΔE can be restatedas memory retention energy.

In addition, there is a case in which, when rearrangement of anoccupation state of an electron orbit, which contributes to spinpolarization, is induced by the application of a voltage, the magneticanisotropy in the direction perpendicular to the film surface can bedirectly controlled also by the voltage application to the side wall, inaccordance with the dependency of the occupation ratio of electronswhich occupy each electron orbit. In this case, it is possible todirectly control not only the interface magnetic anisotropy, but alsothe perpendicular magnetic anisotropy of a material having a highmagnetocrystalline anisotropy.

Next, a description is given of an estimation calculation result of thevariation ratio of perpendicular magnetic anisotropy energy by theapplication of a voltage to the side surface of the storage layer of theMTJ element, with respect to the simplified model.

As illustrated in part (b) of FIG. 1, it is assumed that the radius ofthe magnetic layer 2 is a, and the thickness of the insulation film 4between the magnetic layer 2 and control electrode 6 is b. Asillustrated in part (c) of FIG. 1, it is assumed that the thickness ofthe magnetic layer 2 is t. In this structure, an electrostaticcapacitance C of a cylindrical capacitor, which is constituted by themagnetic layer 2, insulation film 4 and control electrode 6, isexpressed by the following equation (3).

$\begin{matrix}{C = \frac{2{\pi ɛ}_{r}ɛ_{0}t}{\ln( \frac{a + b}{a} )}} & (3)\end{matrix}$where ∈₀=8.85×10⁻¹² (F/m) is a dielectric constant of vacuum, and ∈_(r)is a dielectric constant of the insulation film 4. From equation (3), asurface accumulated charge amount Q, which occurs by applying a voltageV (V) between the magnetic layer 2 and electrode 6, is given by thefollowing equation (4).

$\begin{matrix}{Q = {{CV} = \frac{2\pi\; ɛ_{r}ɛ_{0}t}{\ln( \frac{a + b}{a} )}}} & (4)\end{matrix}$

In addition, the surface area, on which charge accumulation occurs, isS=2πat.

A variation amount K_(s) of magnetic anisotropy energy per unit area ofthe interface is proportional to a charge amount occurring per unit areaof the interface, as indicated by equation (5).

$\begin{matrix}{K_{s} = {c_{s}\frac{Q}{S}}} & (5)\end{matrix}$

Using equation (2), equation (4) and equation (5), a variation amountE_(s) of the magnetization reversal energy barrier by the application ofa voltage to the interface is given by the following equation (6).

$\begin{matrix}{E_{s} = {c_{s}\frac{\pi\; ɛ_{r}ɛ_{0}{tV}}{\ln( \frac{a + b}{a} )}}} & (6)\end{matrix}$

Accordingly, in order to improve the efficiency of the variation ofmagnetic anisotropy energy, it should suffice if the dielectric constant∈_(r) of the insulation film 4 is increased by using the magnetic layer2 of a material with a large constant of proportionality C_(s), and theinsulation film 4 with a high dielectric constant which forms aninterface with this magnetic layer 2.

In addition, E_(s) can be increased by decreasing the denominator ofequation (6). For example, if a has decreased, the variation of bgreatly affects E_(s) relatively. As illustrated in FIG. 2, it isunderstood that, in the case of an MTJ element (magnetic layer 2) with adiameter of 10 nm, for instance, the variation of E_(s) relative to thefilm thickness b of the side-wall insulation film (insulation film 4)sharply increases from b that is 1 nm or less. In the neighborhood ofb=1 nm, E_(s) can greatly be increased by only slightly decreasing b.

On the other hand, the decrease of b in the neighborhood of b=1 nmcauses a decrease in breakdown voltage of the MTJ element. Thus, whenthe stable operation of the MTJ element is considered, it is desirablenot to make thin the side-wall insulation film over the entirety of theperipheral surface of the MTJ element.

[2] MTJ Element

[2-1] Basic Configuration

Referring to FIG. 3 and FIG. 4, an MTJ element of a magnetic memory ofeach of embodiments is described. The magnetic memory of each embodimentincludes at least one memory cell, and this memory cell includes an MTJelement as a memory element.

As illustrated in FIG. 3, an MTJ element 20 includes a bottom electrode21, a magnetic layer 22, a tunnel barrier layer 23, a magnetic layer 24,a top electrode 26, a side-wall insulation film 27, and a controlelectrode 28. The periphery of the MTJ element 20 is covered with aninsulation film 29.

The MTJ element 20 includes a multilayer which includes the magneticlayer 22, tunnel barrier layer 23 and magnetic layer 24. In themultilayer structure 25, the magnetic layer 22 is disposed on the bottomelectrode 21, the tunnel barrier layer 23 is disposed on the magneticlayer 22, and the magnetic layer 24 is disposed on the tunnel barrierlayer 23.

One of the magnetic layer 22 and magnetic layer 24 is a reference layerin which the direction of magnetization is perpendicular to the filmsurface and is fixed, and the other is a storage layer in which thedirection of magnetization is perpendicular to the film surface and isvariable. Here, that the direction of magnetization is fixed means that,when a write current is caused to flow between the bottom electrode 21and top electrode 26, the direction of magnetization is unchanged beforeand after the write current is caused to flow. In addition, that thedirection of magnetization is variable means that, when a write currentis caused to flow between the bottom electrode 21 and top electrode 26,the direction of magnetization is variable before and after the writecurrent is caused to flow. In FIG. 3, the magnetic layer 22 is areference layer, and the magnetic layer 24 is a storage layer. Thus, inthe description below, the magnetic layer 22 is referred to as “fixedlayer 22”, and the magnetic layer 24 is referred to as “storage layer24”. However, conversely to the case shown in FIG. 3, the magnetic layer22 may be a storage layer, and the magnetic layer 24 may be a referencelayer. Besides, the directions of magnetization of the reference layer22 and storage layer 24 may be parallel to the film surface.

The film surface shape (plan-view shape) of the multilayer structure 25may be circular, as illustrated in FIG. 4, or may be an arbitrary shapesurrounded by a closed curve.

The bottom electrode 21 is disposed under the fixed layer 22 of themultilayer structure 25. The bottom electrode 21 has a plan-view shapewhich is different from that of the multilayer structure 25, and has alarger plan-view shape than the multilayer structure 25. However, a partor the entirety of the bottom electrode 21 may have the same plan-viewshape as the multilayer structure 25, and may have a side surface whichagrees with the side surface of the multilayer structure 25.

The top electrode 26 is disposed on the storage layer 24 of themultilayer structure 25. A lower part of the top electrode 26 has thesame plan-view shape as the multilayer structure 25, and has a sidesurface which agrees with the side surface of the multilayer structure25. An upper part of the top electrode 26 has a plan-view shape which isdifferent from the plan-view shape of the multilayer structure 25, andhas a larger plan-view shape than the multilayer structure 25. However,the entirety of the top electrode 26 may have a plan-view shape which isidentical to, or different from, the plan-view shape of the multilayerstructure 25.

The side-wall insulation film 27 is disposed on the side surface of themultilayer structure 25, on the side surface of the lower part of thetop electrode 26, and on the top surface of the bottom electrode 21. Theside-wall insulation film 27 electrically insulates the multilayerstructure 25, top electrode 26 and bottom electrode 21 from the controlelectrode 28. As illustrated in FIG. 4, the side-wall insulation film 27is disposed in a manner to surround the side surface of the storagelayer 24 (multilayer structure 25).

The control electrode 28 is disposed, with the side-wall insulation film27 interposed, on at least the side surface of the storage layer 24. Asillustrated in FIG. 4, the control electrode 28 is disposed in a mannerto surround the side surface of the storage layer 24.

[2-2] Materials

As the storage layer 24, use can be made of, for example, a metal and analloy including a magnetic element selected from among magnetic elementssuch as iron (Fe), cobalt (Co) and nickel (Ni), a Mn-based alloy such asMn—Ga or Mn—Ge, or an oxide (ferrite) including at least one of thesemagnetic elements. As the storage layer 24, use may also be made of alayer formed of a compound or an alloy including rare earth elements,neodymium (Nd), samarium (Sm) and terbium (Tb), and a magnetic element.The storage layer 24 may have a multilayer structure including a firstmagnetic film, a second magnetic film, and a nonmagnetic film disposedbetween the first and second magnetic films.

As the reference layer 22, use can be made of, for example, an alloylayer or an ordered alloy layer including at least one element selectedfrom the group consisting of Fe, Co and Ni as magnetic elements, and atleast one element selected from the group consisting of Pt, Pd, Ru andRe, a Mn-based alloy such as Mn—Ga or Mn—Ge, or a multilayer structurein which a plurality of these layers are stacked. The reference layer 22may have a multilayer structure including a first magnetic film, asecond magnetic film, and a nonmagnetic film disposed between the firstand second magnetic films. In this case, the first and second magneticfilms are magnetically coupled via the nonmagnetic film.

As the tunnel barrier layer 23, for example, a thin film of an oxide,such as MgO or Al₂O₃, can be used.

As the side-wall insulation film 27, use can be made of, for example, alayer of one of an oxide, a nitride and a fluoride, each of whichincludes at least one element selected from the group consisting ofsilicon (Si), aluminum (Al), magnesium (Mg), hafnium (Hf), cerium (Ce),strontium (Sr), tantalum (Ta) and titanium (Ti). It is desirable to usea dielectric with a high dielectric constant as the side-wall insulationfilm 27, in order to enhance the efficiency of control of the energybarrier by the application of a voltage.

[2-3] Thickness of Side-Wall Insulation Film

It is desirable to set a thickness b of the side-wall insulation film 27so as to satisfy both of the following first and second requirements.

As the first requirement, it is desired that a leak current, whichoccurs due to the voltage applied between the control electrode 28 andmultilayer structure 25, be sufficiently lower than a current which iscaused to flow between the top electrode 26 and bottom electrode 21 at atime or write or read of the MTJ element 20. Accordingly, in order tomeet this first requirement, it is desirable that the thickness b of theside-wall insulation film 27 be large to a certain degree.

As the second requirement, it is desired to enhance the efficiency ofcontrol of anisotropy energy of the storage layer 24 by applying avoltage to the control electrode 28, at a time of write or read of theMTJ element 20. Accordingly, in order to meet this second requirement,it is desirable that the thickness b of the side-wall insulation film 27be small.

Although values, which are desirable in order to satisfy both the firstand second requirements, vary depending on the materials of theside-wall insulation film 27, fixed layer 22 and storage layer 24, thethickness of the side-wall insulation film 27 should be set preferablyin a range of 0.5 nm to 10 nm, more preferably in a range of 0.5 nm to 3nm, and still more preferably in a range of 1 nm to 2 nm.

[3] Operations of the Magnetic Memory

Referring to FIG. 3, a description is given of a read operation, a dataretention state and a write operation of the magnetic memory of each ofthe embodiments.

[3-1] Read Operation

When data stored in the MTJ element 20 is read, a read current is causedto flow between the top electrode 26 and bottom electrode 21, and readis executed to determine whether the magnetization of the storage layer24 of the MTJ element 20 is in a parallel (low resistance) state or inan antiparallel (high resistance) state, relative to the magnetizationof the reference layer 22.

Further, when a read current is caused to flow, a voltage is appliedbetween the storage layer 24 and control electrode 28, and the energybarrier in the storage layer 24 is increased, as indicated by the chainline 11 in part (d) of FIG. 1.

The potential of the control electrode 28 is set to be a positivepotential relative to the storage layer 24, for example, in the case ofusing CoFeB with perpendicular magnetization as the material of thestorage layer 24, or in the case of the voltage effect of the samepolarity as this material, that is, in the case of the storage layer 24with such characteristics that when a higher potential is applied to thecontrol electrode 28 than to the storage layer 24, the magneticanisotropy energy in the direction perpendicular to the interfacedecreases and a more stable state occurs in the direction parallel tothe interface, i.e. in the direction perpendicular to the normal of theinterface.

On the other hand, the potential of the control electrode 28 is set tobe a negative potential relative to the storage layer 24, in the case ofusing FePd of a L1₀ structure as the storage layer 24, or in the case ofthe voltage effect of the same polarity as this, that is, in the case ofthe storage layer 24 with such characteristics that when a lowerpotential is applied to the control electrode 28 than to the storagelayer 24, the magnetic anisotropy energy in the direction perpendicularto the interface decreases and a more stable state occurs in thedirection parallel to the interface, i.e. in the direction perpendicularto the normal of the interface.

In this manner, when the read current is caused to flow, such a voltageas to increase the energy barrier ΔE of magnetization reversal in thestorage layer 24 is applied between the storage layer 24 and controlelectrode 28. Thereby, the probability of magnetization reversal of thestorage layer 24 by the read current decreases, and the occurrence ofread disturb can be prevented.

[3-2] Data Retention State

In a data retention state in which neither write nor read is executed inthe MTJ element 20, the potential of the control electrode 28 is set tobe substantially equal to the potential of the storage layer 24, forexample, such that a potential difference falls within 0.1 V. Thissetting is realized, for example, by electrically connecting the controlelectrode 28 and one of the top electrode 26 and bottom electrode 21. Inthis case, the potential energy of the storage layer 24 is in the stateindicated by the solid line 10 in part (d) of FIG. 1. The energy barrierΔE in the data retention state needs to be set to become sufficientlylarge, so that the magnetization of the storage layer 24 may not bereversed due to thermal agitation during a necessary data retention time(e.g. 10 years).

In addition, in the data retention state, as indicated by the chain line11 in part (d) of FIG. 1, the potential of the control electrode 28 maybe set so that the energy barrier ΔE that is necessary for magnetizationreverse may become large. In this case, too, no problem occurs since thedata retention time becomes longer.

[3-3] Write Operation

When data is written to the MTJ element 20, a write current is caused toflow between the top electrode 26 and bottom electrode 21, and themagnetization of the storage layer 24 is reversed by spin-transferwriting.

Further, when the write current is caused to flow, the potential of thecontrol electrode 28 is set such that the storage layer 24 and controlelectrode 28 have substantially equal potentials, for example, such thata potential difference falls within 0.1 V. In this case, the potentialenergy of magnetization of the storage layer 24 becomes approximately ata level as indicated by the solid line 10 in part (d) of FIG. 1. Thus,the energy barrier ΔE that is necessary for magnetization reversalbecomes less than the value at the time of read, and an increase incurrent necessary for write can be prevented.

In addition, when the write current is caused to flow, as indicated bythe chain line 12 in part (d) of FIG. 1, a voltage may be applied to thecontrol electrode 28 so that the energy barrier ΔE necessary formagnetization reverse may become small. In this case, the polarity ofthe voltage becomes opposite to the polarity of the voltage that isapplied at the time of read. Specifically, in the case of using CoFeBwith perpendicular magnetization as the material of the storage layer24, or in the case of the storage layer 24 which exhibits the voltageeffect of the same polarity as this, the potential of the controlelectrode 28 is set to a negative potential relative to the storagelayer 24. On the other hand, in the case of using FePd of the L1₀structure as the storage layer 24, or in the case of the storage layer24 which exhibits the voltage effect of the same polarity as this, thepotential of the control electrode 28 is set to a positive potentialrelative to the storage layer 24.

In this manner, when the write current is caused to flow, such a voltageas to decrease the energy barrier ΔE of magnetization reversal in thestorage layer 24 at the time of write is applied between the storagelayer 24 and control electrode 28. Thereby, the electric current in thespin-transfer writing can be decreased.

In particular, in this write operation, the write current can be furtherdecreased by locally generating a high electric field application regionin the storage layer 24.

[4] First Embodiment

In a first embodiment, in order to locally generate a high electricfield application region in the storage layer 24, a region with a smallfilm thickness b is disposed in the insulation film 27.

[4-1] Configuration

Referring to part (a) of FIG. 5 and part (b) of FIG. 5, theconfiguration of an MTJ element according to the first embodiment isdescribed. Part (a) of FIG. 5 and part (b) of FIG. 5 are cross-sectionalplan views of the storage layer 24 in FIG. 3.

As illustrated in part (a) of FIG. 5 and part (b) of FIG. 5, a side-wallinsulation film 27 is formed on the periphery of the storage layer 24,and a control electrode 28 is formed on the outside of this side-wallinsulation film 27. The side-wall insulation film 27 includes a regionR1 with a film thickness b1 which is large, and a region R2 with a filmthickness b2 which is small. The thickness b2 of the side-wallinsulation film 27 in the region R2 is less than the thickness b1 of theside-wall insulation film 27 in the region R1. Here, when an electricfield was generated by applying a voltage between the storage layer 24and control electrode 28, the electric field intensity in the region R2becomes higher than the electric field intensity in the region R1.Specifically, by the region R2 where the thickness b2 of the side-wallinsulation film 27 is small, a local high electric field region H occursat an interface between the storage layer 24 and side-wall insulationfilm 27, when a voltage is applied between the storage layer 24 andcontrol electrode 28.

In part (a) of FIG. 5, since the number of regions R2 where thethickness b2 of the side-wall insulation film 27 is small is one, asingle high electric field region H is formed. In part (b) of FIG. 5,since the number of regions R2 where the thickness b2 of the side-wallinsulation film 27 is small is two, two high electric field regions Hare formed. Incidentally, reference sign “L” in the Figure indicates aregion where the electric field is lower than in the high electric fieldregion H and a normal electric field is formed in a case in which nofilm thickness distribution is imparted to the side-wall insulation film27.

For example, it is assumed that MgO is used as the side-wall insulationfilm 27, the thickness b1 of the side-wall insulation film 27 in theregion L (region R1) is set at 2 nm, and the thickness b2 of the of theside-wall insulation film 27 in the region H (region R2) is set at 0.9nm. In this case, as is understood from FIG. 2, the variation amountE_(s) of the magnetic reversal energy barrier in the region H becomes 6,and the variation amount E_(s) of the magnetic reversal energy barrierin the region L becomes 3. Thus, in the region H, compared to the regionL, the magnetic reversal energy barrier can be reduced to about ½. As aresult, the write current can be reduced, contributing to energy saving.

In the meantime, as regards the thickness b of the side-wall insulationfilm 27, it is desirable that the difference between the maximum valueand minimum value be 10% or more. In addition, it is desirable that amaximum value Emax of the intensity of the electric field occurringbetween the storage layer 24 and control electrode 28 be greater, by 10%or more, than an average value Eave of the intensity of the electricfield occurring between the storage layer 24 and control electrode 28.

[4-2] Advantageous Effects

As a comparative example, a case is considered in which a thin side-wallinsulation film 27 is disposed over the entire periphery of the storagelayer 24 (for example, the thickness b of the entirety of the side-wallinsulation film 27 is 0.9 nm). In this case, there was an MTJ element 20in which the write energy was successfully reduced. However, such an MTJelement 20 was also observed that short-circuit occurred between thecontrol electrode 28 and multilayer structure 25 due to a pinhole or acrystalline defect occurring in the side-wall insulation film 27, andthere occurred a decrease in yield.

By contrast, in the first embodiment, the side-wall insulation film 27,which is disposed between the multilayer structure 25 and controlelectrode 28, has a film thickness distribution along the periphery ofthe multilayer structure 25. Thus, in the write operation, when avoltage was applied between the storage layer 25 and control electrode28, the high electric field region H is generated in the region R2 wherethe thickness b2 of the side-wall insulation film 27 is small. In thishigh electric field region H, an electric charge is induced at theinterface between the side-wall insulation film 27 and storage layer 24,and the magnetic anisotropy energy of the storage layer 24 in thevicinity of this interface can greatly be lowered. Specifically, bylocally decreasing the magnetic anisotropy energy of the storage layer24, a path, which easily causes reversal of magnetization of the storagelayer 24, can be formed. As a result, by the spin torque acting on thestorage layer 24, which was caused by the write current flowing betweenthe top electrode 26 and bottom electrode 21, the magnetic moment of thestorage layer 24, which started a precession, inclines earlier, and thereception of torque becomes easier. Thereby, the time that is needed formagnetization reversal of the storage layer 24 can be shortened.Therefore, the power consumption in the write operation can be reduced.

In the meantime, a rectangular MTJ element 20 as illustrated in part (a)of FIG. 6 and part (b) of FIG. 6 can also realize the reduction in writeenergy by generating the local high electric field region H. In the caseof a rectangular multilayer structure 25, since a side-wall insulationfilm 27 and a control electrode 28 are formed on the side walls of themultilayer structure 25 in the manufacturing process, the shape of theentirety of the MTJ element 20 is also rectangular. As illustrated inpart (a) of FIG. 6 and part (b) of FIG. 6, the side-wall insulation film27, which includes MgO, includes a region H (b2=0.9 nm) where the filmthickness b2 is small, and a region L (b1=2 nm) with a normal filmthickness b1. In this case, too, the write power can be reduced,compared to the MTJ element 20 in which the thickness b of the side-wallinsulation film 27 is 2 nm over the entirety thereof. In particular, inthe case of the rectangular MTJ element 20, as illustrated in part (b)of FIG. 6, regions He with high electric field intensities exist also atcorner portions of the side-wall insulation film 27. Thus, in the caseof the rectangular MTJ element 20, compared to the circular MTJ element20, a greater effect of the local high electric field can be obtained.

[5] Second Embodiment

In the first embodiment, the thickness b of the insulation film 27 ispartly decreased in order to locally generate the high electric fieldregion H in the storage layer 24. By contrast, in a second embodiment, aplurality of side-wall insulation films with different dielectricconstants are used, and the thickness of an insulation film with a highdielectric constant is partly increased, thereby generating a highelectric field region H. In the meantime, on the presupposition that thethickness (or the distance between the storage layer 24 and controlelectrode 28) of the side-wall insulation film 27 is constant, a greaterelectric field can be applied to the interface of the storage layer 24as the thickness of the film with a greater dielectric constant becomeslarger.

[5-1] Configuration

Referring to part (a) of FIG. 7 to part (c) of FIG. 7, part (a) FIG. 8,and part (b) of FIG. 8, the configuration of an MTJ element according tothe second embodiment is described. Here, although two insulation filmsare used as the side-wall insulation film 27, three or more insulationfilms may be used as the side-wall insulation film 27.

As illustrated in part (a) of FIG. 7 to part (c) of FIG. 7, part (a)FIG. 8 and part (b) of FIG. 8, in the second embodiment, the side-wallinsulation film 27 includes a first insulation film 27 a and a secondinsulation film 27 b. The first insulation film 27 a includes a materialwith a high dielectric constant. The second insulation film 27 bincludes a material with a dielectric constant which is lower than thedielectric constant of the material of the first insulation film 27 a.

On the side surfaces of the storage layer 24, the thickness b of theside-wall insulation film 27 is substantially uniform as a whole, butthe ratio between the thickness of the first insulation film 27 a andthe thickness of the second insulation film 27 b is varied.Incidentally, the thickness b of the side-wall insulation film 27 (thesum of the thickness of the first insulation film 27 a and the thicknessof the second insulation film 27 b) is greater than, for example, thethickness of the tunnel barrier layer 23.

In the example of part (a) of FIG. 7 to part (c) of FIG. 7, all sidesurfaces of the storage layer 24 are surrounded by the two insulationfilms 27 a and 27 b.

In the case of part (a) of FIG. 7, in the side-wall insulation film 27of the two-layer structure, the second insulation film 27 b with a lowdielectric constant is disposed on the storage layer 24 side, and thefirst insulation film 27 a with a high dielectric constant is disposedon the control electrode 28 side. Thus, the first insulation film 27 awith the high dielectric constant is not in direct contact with any oneof the side surfaces of the storage layer 24, and the second insulationfilm 27 b with the low dielectric constant is in direct contact with allside surfaces of the storage layer 24.

The first insulation film 27 a includes a region R1 with a filmthickness b1 which is large, and a region R2 with a film thickness b2which is small. The thickness b1 of the side-wall insulation film 27 inthe region R1 is greater than the thickness b2 of the side-wallinsulation film 27 in the region R2. For example, the region R1 is aregion including a portion with the largest thickness of the firstinsulation film 27 a, and the region R2 is a region including a portionwith the smallest thickness of the first insulation film 27 a.

The second insulation film 27 b includes a region R3 with a filmthickness b3 which is large, and a region R4 with a film thickness b4which is small. The thickness b3 of the side-wall insulation film 27 inthe region R3 is greater than the thickness b4 of the side-wallinsulation film 27 in the region R4. For example, the region R3 is aregion including a portion with the largest thickness of the secondinsulation film 27 b, and the region R4 is a region including a portionwith the smallest thickness of the second insulation film 27 b. Here,for example, the region R2 and region R3 are opposed to each other, andthe region R1 and region R4 are opposed to each other.

The case of part (b) of FIG. 7 differs from the case of part (a) of FIG.7 in that, in the side-wall insulation film 27 of the two-layerstructure, the first insulation film 27 a with the high dielectricconstant is disposed on the storage layer 24 side, and the secondinsulation film 27 b with the low dielectric constant is disposed on thecontrol electrode 28 side. Thus, the first insulation film 27 a with thehigh dielectric constant is in direct contact with all side surfaces ofthe storage layer 24, and the second insulation film 27 b with the lowdielectric constant is not in direct contact with any one of the sidesurfaces of the storage layer 24.

The case of part (c) of FIG. 7 differs from the case of part (b) of FIG.7 in that the thickness of the second insulation film 27 b issubstantially uniform.

In the examples of part (a) of FIG. 7 to part (c) of FIG. 7, when avoltage is applied between the storage layer 24 and control electrode28, an electric charge tends to be easily accumulated in the region R1with the large film thickness of the first insulation film 27 a with thehigh dielectric constant, and a high electric field region H is formednear this region R1.

In the meantime, in the cases of part (a) of FIG. 7 and part (b) of FIG.7, in the region H, the thickness b1 of the first insulation film 27 ais about 2 nm, the thickness b4 of the second insulation film 27 b is0.5 nm or less, and the thickness b of the side-wall insulation film 27is 2.5 nm or less. On the other hand, in the region L, the thickness b2of the first insulation film 27 a is about 1 nm, the thickness b3 of thesecond insulation film 27 b is about 2 nm, and the thickness b of theside-wall insulation film 27 is about 3 nm.

In examples of part (a) of FIG. 8 and part (b) of FIG. 8, parts of theside surface of the storage layer 24 are surrounded by two insulationfilms 27 a and 27 b, but the other parts of the side surface of thestorage layer 24 are surrounded by one insulation film 27 a or 27 b.Thus, the side surface of the storage layer 24 includes a part which isin direct contact with the first insulation film 27 a with a highdielectric constant, and a part which is in direct contact with thesecond insulation film 27 b with a low dielectric constant.

In the case of part (a) of FIG. 8, in the part of the side-wallinsulation film 27 of the two-layer structure, the second insulationfilm 27 b with the low dielectric constant is disposed on the storagelayer 24 side, and the first insulation film 27 a with the highdielectric constant is disposed on the control electrode 28 side.Furthermore, a region H of the first insulation film 27 a, which isdisposed in direct contact with the side surface of the storage layer24, and a region L of the second insulation film 27 b, which is disposedin direct contact with the side surface of the storage layer 24, areformed.

In the case of part (b) of FIG. 8, in the part of the side-wallinsulation film 27 of the two-layer structure, the first insulation film27 a with the high dielectric constant is disposed on the storage layer24 side, and the second insulation film 27 b with the low dielectricconstant is disposed on the control electrode 28 side. Furthermore, aregion H of the first insulation film 27 a, which is disposed in directcontact with the side surface of the storage layer 24, and a region L ofthe second insulation film 27 b, which is disposed in direct contactwith the side surface of the storage layer 24, are formed.

In the meantime, the MTJ elements 20 of part (a) of FIG. 7 and part (b)of FIG. 7 are of the rectangular type, and the MTJ elements 20 of part(a) of FIG. 8 and part (b) of FIG. 8 are of the circular type.Alternatively, the MTJ elements 20 of part (a) of FIG. 7 and part (b) ofFIG. 7 may be of the circular type, and the MTJ elements 20 of part (a)of FIG. 8 and part (b) of FIG. 8 may be of the rectangular type. Inaddition, a plurality of high electric field regions H may be disposed.

[5-2] Materials

Examples of the material with a high dielectric constant of the firstinsulation film 27 a include perovskite-based materials such as SrRuO₃,SrIrO₃, BaTiO₃, and StTiO₃.

Examples of the material with a low dielectric constant of the secondinsulation film 27 b include magnesium oxide, aluminum oxide, aluminumnitride, silicon oxide, and silicon nitride.

In the case of very thin films on the order of nanometers, films of alayer structure tend to become uniform, with these materials of theinsulation films 27 a and 27 b.

In the case of part (a) of FIG. 7, for example, the second insulationfilm 27 b is MgO with a dielectric constant of about 10, and the firstinsulation film 27 a is strontium titanate with a dielectric constant ofabout 300. Taking into account the movement of oxygen to a magneticbody, MgO that is stable as an oxide is used for the second insulationfilm 27 b that is primitively in contact with the storage layer 24.Thus, a design with a wide margin for a thermal process can be made, andthere is a merit in terms of cost.

In the meantime, in the case of an insulation film that comes in directcontact with the storage layer 24, it is desirable to form theinsulation film of particles with thermal energy, for example, by MBE(Molecular Beam Epitaxy), vacuum evaporation, ALD (Atomic LayerDeposition), CVD (Chemical Vapor Deposition), etc.

[5-3] Advantageous Effects

In the second embodiment, the insulation films 27 a and 27 b withdifferent dielectric constants are used as the side-wall insulation film27. Thus, in the write operation, when a voltage is applied between thestorage layer 24 and control electrode 28, an electric charge tends tobe easily accumulated in the region with a large thickness of the firstinsulation film 27 a with the high dielectric constant, under thecondition that the film thickness of the side-wall insulation film 27(or the distance between the storage layer 24 and control electrode 28)is constant, and the high electric field region H is locally formed.Thereby, like the first embodiment, the write current can be reduced bylocally decreasing the magnetic anisotropy energy of the storage layer24.

[6] Third Embodiment

In a third embodiment, the arrangement of high electric field regions Hin a memory cell array is described.

[6-1] Arrangement of High Electric Field Regions.

Referring to FIG. 9 and FIG. 10, a description is given of thearrangement of high electric field regions H in a memory cell array of amagnetic memory according to the third embodiment.

As is illustrated in FIG. 9, in the magnetic memory, the plural MTJelements 20 of the first and second embodiment are arranged in an array,and constitute a memory cell array 100. Here, local high electric fieldregions H are located at relatively identical positions. In FIG. 9, thehigh electric field regions H are uniformly arranged on the left side ofthe respective MTJ elements 20 on the sheet surface of FIG. 9.

In the third embodiment, when the MTJ elements 20 of the firstembodiment are used, the positions of the thin regions of the side-wallinsulation films 27 are uniformly arranged within the memory cell array100.

In the third embodiment, when the MTJ elements 20 of the secondembodiment are used, the positions of the thick regions of theinsulation films 27 b with the high dielectric constant are uniformlyarranged within the memory cell array 100.

As illustrated in FIG. 10, when attention is paid to two MTJ elements 20a and 20 b in the memory cell array 100, in the case where an electricfield occurs by applying a voltage between the storage layer 24 andcontrol electrode 28, the electric field intensity at a position P1 inthe MTJ element 20 a is higher than the electric field intensity at aposition P2, and the electric field intensity at a position P1 in theMTJ element 20 b is higher than the electric field intensity at aposition P2. In addition, the film thickness at the position P1 of theside-wall insulation film 27 of the MTJ element 20 a is less than thefilm thickness at the position P2, and the film thickness at theposition P1 of the side-wall insulation film 27 of the MTJ element 20 bis less than the film thickness at the position P2.

Here, the position P1 in the MTJ element 20 a is a position of crossingbetween the side-wall insulation film 27 and a line segment Xa having afirst angle θ1 to a straight line X connecting a center point C1 of theMTJ element 20 a and a center point C2 of the MTJ element 20 b, the linesegment Xa connecting the center point C1 and a first peripheral portionE1 of the MTJ element 20 a. In addition, the position P2 in the MTJelement 20 a is a position of crossing between the side-wall insulationfilm 27 and a line segment Xb having a second angle θ2 to the straightline X, which is different from the first angle θ1, and connecting thecenter point C1 and a second peripheral portion E2 of the MTJ element 20a.

Similarly, the position P1 in the MTJ element 20 b is a position ofcrossing between the side-wall insulation film 27 and a line segment Xahaving a first angle θ1 to the straight line X, and connecting thecenter point C2 and a first peripheral portion E1 of the MTJ element 20b. In addition, the position P2 in the MTJ element 20 b is a position ofcrossing between the side-wall insulation film 27 and a line segment Xbhaving a second angle θ2 to the straight line X, and connecting thecenter point C2 and a second peripheral portion E2 of the MTJ element 20b.

The center points C1 and C2 of the MTJ elements 20 a and 20 b areequivalent to the centers of gravity of the shapes of the MTJ elements20 a and 20 b. In addition, the first angle θ1 and second angle θ2 areangles as viewed in the same direction (e.g. clockwise) with referenceto the straight line X.

[6-2] Forming Method (1) of the Side-Wall Insulation Film

In a forming method (1) of the side-wall insulation film 27 of thepresent embodiment, evaporation deposition is used.

Referring to FIG. 11 and FIG. 12, a description is given of the formingmethod (1) of the side-wall insulation film 27 of the MTJ element 20according to the third embodiment.

After the multilayer structure 25 of the MTJ element 20 is processed ina cylindrical shape, a wafer is disposed within a film-forming apparatus50 of the side-wall insulation film 27 as illustrated in FIG. 11.Evaporation sources 51N, 51E, 51S and 51W of the side-wall insulationfilm 27 are disposed in four directions of the memory cell array 100.Using this film-forming apparatus 50, the side-wall insulation film 27is formed obliquely with respect to the substrate. Thereby, theside-wall insulation film 27 with few defects can be formed on the sidewall of the multilayer structure 25.

In the film forming step, for example, in the evaporation sources 51N,51E, 51S and 51W which are disposed in the four directions of the memorycell array 100, the density of side-wall film particles, which aregenerated from the evaporation sources 51N, 51S and 51W, is set to below, and the density of side-wall film particles, which are generatedfrom the evaporation source 51E, is set to be high. Thereby, theside-wall insulation film 27 on the evaporation source 51W side can beformed to have a small thickness.

In addition, as illustrated in FIG. 12, in order to make uniform thefilm thickness distribution of the side-wall insulation film 27, thewafer 52 is moved. At this time, by moving the substrate in the X-Ydirection (E-W direction, N-S direction), the side-wall insulation film27 can be formed with the same film thickness distribution around thestorage layer 24, while the uniformity within the wafer plane is beingmaintained.

[6-3] Forming Method (2) of the Side-Wall Insulation Film

In a forming method (2) of the side-wall insulation film 27 of thepresent embodiment, ion beam etching is used.

Referring to FIG. 13 to FIG. 15, a description is given of the formingmethod (2) of the side-wall insulation film 27 of the MTJ element 20according to the third embodiment.

To start with, as illustrated in part (a) of FIG. 13 and part (a) ofFIG. 14, the multilayer structure 25, bottom electrode 21 and topelectrode 26 of the MTJ element 20 are processed by using an ion beam.

Then, as illustrated in part (b) of FIG. 13 and part (b) of FIG. 14, aside-wall insulation film 27 is uniformly formed around the multilayerstructure 25, bottom electrode 21 and top electrode 26 by using MBE. Atthis time, it is desirable to form the side-wall insulation film 27 soas to have a greater thickness at a bottom portion btm of the multilayerstructure 25 than at a side wall portion sw of the multilayer structure25.

Next, as illustrated in part (c) of FIG. 13 and part (c) of FIG. 14, theside wall portion sw of the side-wall insulation film 27 is mainlyetched in a certain direction by using an ion beam, and the thickness ofthe side-wall insulation film 27 is partly reduced. At this time, by theion beam etching, not only the side wall portion sw of the side-wallinsulation film 27, but also the bottom portion btm of the side-wallinsulation film 27 is etched to some degree. However, since the bottomportion btm of the side-wall insulation film 27 is thicker than the sidewall portion sw of the side-wall insulation film 27, the bottomelectrode 21 is not exposed.

Subsequently, as illustrated in part (d) of FIG. 13 and part (d) of FIG.14, a metal film, which becomes the control electrode 28, is formed onthe side-wall insulation film 27, and this metal film is processed inthe shape of the control electrode 28.

In this method (2), the side-wall insulation film 27 is etched by usingan ion beam, from only one direction of the side wall of the multilayerstructure 25. In this case, as illustrated in FIG. 15, the ion beam isradiated from a certain direction (0 o'clock direction in FIG. 15), andthe uniformity in the wafer plane is secured while the wafer 52 is beingmoved in the X-Y direction.

[6-4] Advantageous Effects

In the third embodiment, in the memory cell array of the magneticmemory, the local high electric field regions H of the MTJ elements 20are located relatively at the same position. Thereby, at a time ofwrite, a high electric field is applied in a certain direction to allMTJ elements 20 within the memory cell array 100, and magneticinteractions among the MTJ elements 20 become uniform. Thus, themagnetization reversal of the storage layer 24 of each MTJ element 20tends to easily occur in the same direction. Thereby, the write currentcan be reduced, and a variance in write current can be suppressed.

[7] Fourth Embodiment

In a fourth embodiment, the control electrode 28 of the MTJ element 20is described with reference to part (a) of FIG. 16, part (b) of FIG. 16,and FIG. 17.

The control electrode 28 in each embodiment is used in order to apply avoltage, and is not configured to make an electric current to flow tocause energy consumption. Thus, there is no need to reduce theelectrical resistance of the control electrode 28. Therefore, the degreeof freedom of the shape of the control electrode 28 is high.

For example, as illustrated in part (a) of FIG. 16, the cross-sectionalshape of the control electrode 28 may be triangular. In this case, thewidth of the control electrode 28 can be reduced. Specifically, sincethe control electrode 28 can be made thin, it is possible to reduce theeffect of anisotropy induction by the compressive stress or tensilestress on the multilayer structure 25 by the control electrode 28. Thus,the effect by the high electric field region H becomes easier to occur.

Besides, as illustrated in part (b) of FIG. 16, the cross-sectionalshape of the control electrode 28 may be rectangular, if the stress onthe multilayer structure 25 by the control electrode 28 can becontrolled.

In addition, as illustrated in FIG. 17, the film thickness of thecontrol electrode 28 may have a distribution. Even if the film thicknessof the control electrode 28 has such a distribution, there ispractically no problem since an electric field can be applied. If thedegree of freedom of the shape of the control electrode 28 is high, aprocessing design margin increases, and there is a merit in terms ofcost. In FIG. 17, a film thickness d of the control electrode 28 in thesame region as the region H where the side-wall insulation film 27 isthin may be decreased. Thereby, in the region H, since the compressivestress on the multilayer structure 25 by the control electrode 28 can becontrolled, the effect by the high electric field region H can beenhanced.

[8] Fifth Embodiment

Referring to FIG. 18, a description is given of the thickness of aside-wall insulation film 27 of an MTJ element 20 according to a fifthembodiment.

As illustrated in FIG. 18, a thickness bs of the side-wall insulationfilm 27 on the side surface of the storage layer 22 is less than athickness br of the side-wall insulation film 27 on the side surface ofthe reference layer 24.

In the meantime, in the fifth embodiment, like the above-describedembodiments, the thickness b of the side-wall insulation film 27 has afilm thickness distribution along the periphery of the side surface ofthe storage layer 24, so that the high electric field region H maylocally occur in the storage layer 24. In addition, the electric fieldintensity of the high electric field region H on the side surface of thestorage layer 24 is higher than the electric field intensity of the highelectric field region H on the side surface of the reference layer 22.Furthermore, the ratio between the maximum value and minimum value ofthe thickness b of the side-wall insulation film 27 on the side surfaceof the storage layer 24 is greater than the ratio between the maximumvalue and minimum value of the thickness b of the side-wall insulationfilm 27 on the side surface of the reference layer 22.

As described above, in the fifth embodiment, the thickness bs of theside-wall insulation film 27 on the side surface of the storage layer 24is set to be less than the thickness br of the side-wall insulation film27 on the side surface of the reference layer 22. Thereby, the action ofthe electric field can strongly be exerted on the storage layer 24 towhich the voltage is to be properly applied. It is thus possible tosuppress a write error, a read error, and a decrease in yield byshort-circuit to the control electrode 28 due to a pinhole.

[9] Sixth Embodiment

In a sixth embodiment, a description is given of a case in which the MTJelement of each of the above-described embodiments is applied to amagnetic memory. An example of the magnetic memory is an MRAM.

[9-1] Memory Cell

Referring to part (a) of FIG. 19 to part (c) of FIG. 19, part (a) ofFIG. 20, and part (b) of FIG. 20, memory cells of the magnetic memoryaccording to the sixth embodiment are described. Part (a) of FIG. 19 isa cross-sectional view taken along line A-A in part (b) of FIG. 19. Part(b) of FIG. 19 is a cross-sectional view taken along line B-B in part(a) of FIG. 19. Part (c) of FIG. 19 is a cross-sectional view takenalong line C-C in part (a) of FIG. 19.

As illustrated in part (a) of FIG. 19 to part (c) of FIG. 19, themagnetic memory includes a plurality of memory cells which are arrangedin a matrix of, for example, four rows×four columns. The respectivememory cells include, as memory elements, MTJ elements 20 ₁₁, 20 ₁₂, 20₂₁ and 20 ₂₂ which were described in the embodiments. Each MTJ element20 _(ij) (i=1, 2, j=1, 2) includes a multilayer structure 25A in which abottom electrode 21, a fixed layer 22, a tunnel barrier layer 23, astorage layer 24, and a top electrode 26 are stacked in the named order.The periphery of the multilayer structure 25A is covered with aside-wall insulation film 27, and a control electrode 28 is disposed onthe outer periphery of the side-wall insulation film 27. In each MTJelement 20 _(ij) (i=1, 2, j=1, 2), the control electrode 28 is disposedin a manner to surround the side surface of the storage layer 24, withthe side-wall insulation film 27 being interposed. In addition, thecontrol electrodes 28 of the MTJ elements arranged on the same row, forexample, the MTJ elements 20 ₁₁ and 20 ₁₂, are electrically connectedand serve as a common control line.

As illustrated in part (a) of FIG. 20, bit lines BL<t> and BL<t+1> areconnected to the top electrodes 26 of the MTJ elements 20 ₁₁ to 20 ₂₂.Here, the bit line BL<t> is connected to the top electrodes 26 of theMTJ element 20 ₁₁ and MTJ element 20 ₂₁. The bit line BL<t+1> isconnected to the top electrodes 26 of the MTJ element 20 ₁₂ and MTJelement 20 ₂₂.

On the other hand, control lines EL<s> and EL<s+1> are disposed in adirection crossing the bit lines. Here, the control line EL<s> isconnected to the control electrodes 28 which are disposed on the sidesurfaces of the MTJ elements 20 ₁₁ and 20 ₁₂, with the side-wallinsulation films 27 interposed. The control line EL<s+1> is connected tothe control electrodes 28 which are disposed on the side surfaces of theMTJ elements 20 ₂₁ and 20 ₂₂, with the side-wall insulation films 27interposed.

As illustrated in part (b) of FIG. 20, in a memory cell 40 including theMTJ element 20 ₁₁, the bottom electrode 21 of the MTJ element 20 ₁₁ isconnected to one of a source and a drain of a select transistor 30. Theother of the source and drain of the select transistor 30 is connectedto a bit line bBL<t>. In addition, a word line WL<s> is connected to thegate of the select transistor 30. The word line WL<s> is disposed inparallel to the control line EL<s>. Incidentally, although part (b) ofFIG. 20 is a view illustrating, by way of example, the memory cell 40including the MTJ element 20 ₁₁, the other memory cells 40 have the sameconfiguration.

[9-2] Potentials of the Bit Lines and Control Lines

Referring to FIG. 21, a description is given of an example of potentialsetting of bit lines and control lines at a time of selecting the MTJelement 20 ₁₁ illustrated in part (a) of FIG. 20 and executing read andwrite. Here, as an example of the potential setting, it is assumed thatthe bit line potential at a time of write is 0.5 V, and the bit linepotential at a time of read is 0.3 V.

As described above, there is a method in which the energy barrier isincreased and the direction of magnetization of the storage layer 24 isstabilized by applying a voltage between the storage layer 24 andcontrol electrode 28. As regards the application voltage in this method,the following two cases can be thought. In a first case, like a magneticlayer of CoFeB or the like, the potential of the control electrode 28 isset to be higher than the potential of the storage layer 24. In a secondcase, like a magnetic layer of FePd, the potential of the controlelectrode 28 is set to be lower than the potential of the storage layer24. The setting of potentials is different between the first case andthe second case. The two cases will individually be described below.

To begin with, a description is given of the case (first case) in which,like a magnetic layer of CoFeB or the like, the potential of the controlelectrode 28 is set to be higher than the potential of the storage layer24.

At a time of write, the control line EL<s+1> is set at a high potential(e.g. 1.5 V), and the magnetization of the storage layer 24 of theconnected MTJ element is stabilized. On the other hand, the control lineEL<s>, to which the selected cell is connected, is set at a potentialwhich is substantially equal to, or lower than, the potential (e.g. 0.5V) of the storage layer 24 of the MTJ element 20 ₁₁ to which write isexecuted. Thereby, it is possible to prevent an increase in writecurrent due to an increase of the energy barrier of the storage layer 24of the MTJ element 20 ₁₁. In the state in which this setting has beenmade, a pulse voltage is applied to the bit line BL<t>, and write isexecuted to the MTJ element 20 ₁₁. During this time, the potential ofthe bit line BL<t+1> is fixed at 0 V. Thereby, the MTJ element 20 ₁₂ isalso kept in the state in which the potential of the control electrode28 is higher than the potential of the storage layer 24, and erroneouswrite can be prevented.

At a time of read, both the control line EL<s> and the control lineEL<s+1> are set at a high potential (e.g. 1.5 V). In this state, a readpulse potential is applied to the bit line BL<t>, and the magnetizationstate of the storage layer 24 of the MTJ element 20 ₁₁ is sensed by asense amplifier. During this time, the potential of the bit line BL<t+1>is fixed at 0 V.

Next, a description is given of the case (second case) in which, like amagnetic layer of FePd, the potential of the control electrode 28 is setto be lower than the potential of the storage layer 24.

At a time of write, the control line EL<s+1> is set at 0 V, anderroneous write to an unselected cell is prevented. The control lineEL<s> is set at a potential which is equal to, or higher than, thepotential (e.g. 0.5 V) of the storage layer 24 of the memory cell towhich write is executed, and an increase in reversal current isprevented. In this state, a write voltage pulse (e.g. 0.5 V) is appliedto the bit line BL<t>, and write is executed. During this time, the bitline BL<t+1> is kept at a high potential (e.g. 1.5 V).

At a time of read, both the control line EL<s> and the control lineEL<s+1> are set at 0 V. A read voltage (e.g. 0.3 V) is applied to thebit line BL<t>, and the magnetization state of the MTJ element 20 ₁₁ issensed by the sense amplifier. By this operation, read disturb can beavoided. During this time, the bit line BL<t+1> is kept at a highpotential (e.g. 1.5 V).

In the meantime, in the above description, the voltage that is necessaryat a time of write is different between the write at a time of settingthe magnetization of the storage layer in the MTJ element in a parallelstate relative to the magnetization of the reference layer, and thewrite at a time of setting the magnetization of the storage layer in theMTJ element in an antiparallel state relative to the magnetization ofthe reference layer. However, only an example is illustrated for thepurpose of simple description, and a detailed description is omitted.Specifically, in this example, write for setting a parallel state isexecuted to an MTJ element in which a storage layer is located on a sideopposite to a select transistor with respect to a reference layer. Infact, potentials of respective bit lines and control lines are adjustedin accordance with conditions of write and read.

[9-3] Magnetic Memory

Referring to FIG. 22, the configuration of the magnetic memory accordingto the sixth embodiment is described.

As illustrated in FIG. 22, memory cells 40 in a memory cell array 100are connected to first bit lines (conductive lines) BL<t> and BL<t+1>,second bit lines (conductive lines) bBL<t> and bBL<t+1>, word lines(conductive lines) WL<s> and WL<s+1>, and control lines (conductivelines) EL<s> and EL<s+1>. Incidentally, the memory cells 40 are the sameas the memory cell 40 shown in part (b) of FIG. 20.

The first bit lines BL<t> and BL<t+1> are connected to a write circuit120 and a read circuit 130 via a bit line select circuit 110. The bitline select circuit 110 includes switch elements (FET) 112<t> and112<t+1> which are ON/OFF controlled by control signals Ayn<t> andAyn<t+1>.

The write circuit 120 includes switch elements (FET) 122 a and 122 bwhich are ON/OFF controlled by control signals SRCn and SNKn.

The read circuit 130 includes a switch element (FET) 130 a which isON/OFF controlled by a control signal SRCr, and a sense amplifier 130 b.

The second bit lines bBL<t> and bBL<t+1> are connected to a writecircuit 125 and a read circuit 135 via a bit line select circuit 115.The bit line select circuit 115 includes switch elements (FET) 117<t>and 117<t+1> which are ON/OFF controlled by control signals Ays<t> andAys<t+1>.

The write circuit 125 includes switch elements (FET) 127 a and 127 bwhich are ON/OFF controlled by control signals SRCs and SNKs.

The read circuit 135 includes a switch element (FET) 135 a which isON/OFF controlled by a control signal SNKr.

The word lines WL<s> and WL<s+1> are connected to a word line selectcircuit 140. The word line select circuit 140 drives the word linesWL<s> and WL<s+1> by control signals Ax<s> and Ax<s+1>.

The control lines EL<s> and EL<s+1> are connected to a control lineselect circuit 150. The control line select circuit 150 drives thecontrol lines EL<s> and EL<s+1> by control signals Bx<s> and Bx<s+1>.

A control circuit 160 generates the control signals SRCn, SNKn, SRCs,SNKs, SRCr and SNKr.

A decoder 170 generates control signals Ayn, Ays, Ax and Bx. It shouldbe noted, however, that the control signals Ayn, Ays, Ax and Bxcomprehensively represent all corresponding control signals.

As has been described above, according to the embodiments, in the writeoperation, the energy barrier, which is necessary for reversal ofmagnetization of the storage layer 24, can be controlled with highefficiency. Thereby, the write current can be reduced.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A magnetic memory comprising a memory cell arrayin which magnetic memory elements are disposed in an array, each of themagnetic memory elements comprising: a first magnetic layer; a secondmagnetic layer; a nonmagnetic layer disposed between the first magneticlayer and the second magnetic layer; an electrode disposed on a sidesurface of the first magnetic layer; and a first insulation layerdisposed between the first magnetic layer and the electrode, wherein themagnetic memory elements in the memory cell array comprise a firstmagnetic memory element and a second magnetic memory element, adielectric constant at a first position in the first magnetic memoryelement is higher than a dielectric constant at a second position in thefirst magnetic memory element, and a dielectric constant at a thirdposition in the second magnetic memory element is higher than adielectric constant at a fourth position in the second magnetic memoryelement, when an electric field occurs by applying a voltage between thefirst magnetic layer and the electrode, the first position is a positionof crossing between the first insulation layer and a line segment havinga first angle to a straight line connecting a first center point of thefirst magnetic memory element and a second center point of the secondmagnetic memory element, the line segment connecting the first centerpoint and a first peripheral portion of the first magnetic memoryelement, the second position is a position of crossing between the firstinsulation layer and a line segment having a second angle to thestraight line, which is different from the first angle, and connectingthe first center point and a second peripheral portion of the firstmagnetic memory element, the third position is a position of crossingbetween the first insulation layer and a line segment having the firstangle to the straight line, and connecting the second center point and athird peripheral portion of the second magnetic memory element, and thefourth position is a position of crossing between the first insulationlayer and a line segment having the second angle to the straight line,and connecting the second center point and a fourth peripheral portionof the second magnetic memory element.
 2. The element of claim 1,wherein the first insulation layer comprises at least one selected fromthe group consisting of an oxide, a nitride and a fluoride, each ofwhich comprises at least one selected from the group consisting ofsilicon, aluminum, magnesium, hafnium, cerium, strontium, tantalum andtitanium.
 3. The element of claim 1, wherein the first insulation layercomprises at least one selected from the group consisting of SrRuO₃,SrIrO₃, BaTiO₃, and StTiO₃, magnesium oxide, aluminum oxide, aluminumnitride, silicon oxide, and silicon nitride.
 4. The element of claim 1,wherein the first insulation layer is disposed on a side surface of thesecond magnetic layer, and a film thickness of the first insulationlayer at the first magnetic layer is less than a film thickness of thefirst insulation layer at the second magnetic layer.
 5. A magneticmemory comprising a memory cell array in which magnetic memory elementsare disposed in an array, each of the magnetic memory elementscomprising: a first magnetic layer; a second magnetic layer; anonmagnetic layer disposed between the first magnetic layer and thesecond magnetic layer; an electrode disposed on a side surface of thefirst magnetic layer; and a first insulation layer disposed between thefirst magnetic layer and the electrode, wherein the magnetic memoryelements in the memory cell array comprise a first magnetic memoryelement and a second magnetic memory element, a film thickness at afirst position in the first insulation layer of the first magneticmemory element is less than a film thickness at a second position in thefirst insulation layer of the first magnetic memory element, and a filmthickness at a third position in the first insulation layer of thesecond magnetic memory element is less than a film thickness at a fourthposition in the first insulation layer of the second magnetic memoryelement, the first position is a position of crossing between the firstinsulation layer and a line segment having a first angle to a straightline connecting a first center point of the first magnetic memoryelement and a second center point of the second magnetic memory element,the line segment connecting the first center point and a firstperipheral portion of the first magnetic memory element, the secondposition is a position of crossing between the first insulation layerand a line segment having a second angle to the straight line, which isdifferent from the first angle, and connecting the first center pointand a second peripheral portion of the first magnetic memory element,the third position is a position of crossing between the firstinsulation layer and a line segment having the first angle to thestraight line, and connecting the second center point and a thirdperipheral portion of the second magnetic memory element, and the fourthposition is a position of crossing between the first insulation layerand a line segment having the second angle to the straight line, andconnecting the second center point and a fourth peripheral portion ofthe second magnetic memory element.
 6. The element of claim 5, whereinthe first insulation layer comprises at least one selected from thegroup consisting of an oxide, a nitride and a fluoride, each of whichcomprises at least one selected from the group consisting of silicon,aluminum, magnesium, hafnium, cerium, strontium, tantalum and titanium.7. The element of claim 5, wherein the first insulation layer comprisesat least one selected from the group consisting of SrRuO₃, SrIrO₃,BaTiO₃, and StTiO₃, magnesium oxide, aluminum oxide, aluminum nitride,silicon oxide, and silicon nitride.
 8. The element of claim 5, whereinthe first insulation layer is disposed on a side surface of the secondmagnetic layer, and a film thickness of the first insulation layer atthe first magnetic layer is less than a film thickness of the firstinsulation layer at the second magnetic layer.